This invention relates to a process for the conversion of a mixture of halosilanes to alkoxysilanes, and more particularly, the present invention relates to a process for separating at least two alkoxysilanes prepared by the co-alkoxylation of at least two halosilanes which are difficult to separate.
The demand for alkoxysilanes, such as, dimethyldimethoxysilane and methyltrimethoxysilane, is increasing because the alkoxysilanes are now being used as important intermediates in the silicone industry for the production of various fluids and elastomers. Heretofore, it has been difficult to obtain high purity products from commercially-prepared alkoxysilanes because it is difficult to separate the halosilanes from which the alkoxysilanes are prepared. In the production of the halosilanes, it is frequently difficult and economically unfeasible to separate halosilanes, such as, dimethyldichlorosilane and methyltrichlorosilane, because of their close boiling points. Methyltrichlorosilane boils at 66.degree. C., and dimethyldichlorosilane boils at 70.degree. C., and when the two fractions appear in the same product in the manufacture of organosilicon halides, these close-boiling fractions are separated with difficulty in long distillation columns because of the close boiling points. For example, in the separation of dimethyldichlorosilane from methyltrichlorosilane, a very long distillation column is employed with a reflux ratio as high as 100:1. Thus, the separation of the dimethyldichlorosilane from methyltrichlorosilane is an energy intensive process.
There are many processes for preparing alkoxysilanes from halosilanes, such as, chlorosilanes, with such alkoxylating agents as hydroxyl-containing aliphatic compounds in a reflux column maintained at an elevated temperature and equipped with a reflux condenser. In certain of the prior art processes, either gaseous alcohol and gaseous chlorosilane are introduced into the column in a stoichiometric ratio from opposite points in the longitudinal sides of the column approximately one-half way up the column, or the gaseous halosilane is introduced from below into the column in counter-current flow to the alcohol, which may contain water, and which flows downward in the column.
One continuous process for preparing alkoxysilanes is described in U.S. Pat. No. 3,792,071 where chlorosilanes are reacted with optionally substituted alcohols and, if desired, water, to form polyalkoxysilanes, in a column provided with a reflux condenser and kept at an elevated temperature, wherein (a) the chlorosilane is introduced at the head of the column, and the alcohol is introduced in the gaseous form from below or at a point in the lowest one-third length of the column, and water, if used, is introduced at any desired point of the column; (b) the reaction product is removed from the column at a point below the point of introduction of the alcohol or at the lower end; (c) for at least two-thirds of the zone between the inlet of the alcohol and the inlet of the silane into the column, the column is maintained, over its entire internal cross-section at a temperature at least 0.5.degree. C. above the boiling point of the particular alcohol at the particular pressure prevailing in the column; and (d) during the reaction excess alcohol boiling under reflux is constantly present at the head of the column.
In U.S. Pat. No. 3,651,117, halosilanes are also esterified by mixing the halosilane and a sufficient amount of alcohol in a reaction zone wherein at least 80% of the halogen groups of the halosilane are esterified by reaction with the alcohol, the reaction zone having a temperature sufficently high enough to maintain said halosilane, alcohol and the highest boiling esterified silane reaction product in the vapor state, and removing the products of the reaction from the reaction zone while in the vapor state. In U.S. Pat. No. 4,039,567, an alkoxysilane of the formula, R.sub.4-n-m Si(OR').sub.n Cl.sub.m, where R is an alkyl radical or hydrogen; R' is an alkyl radical; m is 0 to 3; n is 1 to 4; and n+M is equal to or less than 4; is prepared by the esterification of a chlorosilane of the formula, R.sub.4-n SiCl.sub.n, with an alcohol by continuously charging liquid alcohol and liquid chlorosilane from a separate source into a distillative reaction zone having a head portion and a sump portion, maintaining the head portion at a temperature sufficient for the esterification, continuously distilling off gaseous hydrogen chloride formed during the esterification while maintaining the resultant reaction mixture in the sump at its boiling point and continuously separating liquid alkoxysilane from the sump. In another process for the esterification of an organochlorosilane by feeding alcohol into a chlorosilane maintained within a reaction zone without the alcohol contacting the chlorosilane in the gas phase wherein the esterification is performed step-wise with the extraction of hydrogen chloride which has formed as by-product, it is disclosed in U.S. Pat. No. 4,228,092, that an organochlorosilane of the formula, R.sub.a R.sub.b SiCl.sub.4-a-b wherein R is typically a halogen-substituted alkyl radical, a is 0-2, b is 1 or 2, and a+b is a maximum of 3, is employed in at least a final esterification step which is performed with the addition of heat.
Although the foregoing prior art processes are used for preparing alkoxysilanes from chlorosilanes by conventional alkoxylation, it is desirable to co-alkoxylate mixtures of halosilanes, and especially close-boiling halosilanes, to produce high purity alkoxysilanes from mixtures of halosilanes, especially for applications requiring the high purity alkoxysilane monomers. It is also desirable to eliminate the present techniques required to prepare first the high purity halosilane by energy intensive processes, such as the use of long distillation columns employing high reflux ratios, wherein the high purity alkoxysilane is prepared from a high purity halosilane.